For example, let's say I create a square with Grid SOP.

I can access the P attribute with an Attribute Wrangler running over primitives:


However, the P attribute isn't on primitives. It's on points! Why is the above code working? And does it always work for every attribute other than P?

Primitives have a hidden, read-only P attribute too. The output you're seeing is the position of the prims, not the points. You can verify this by running it on a grid with a single prim.